The HumSat project is an international educational initiative for building a constellation of nanosatellites providing worldwide communication capabilities to areas without infrastructure. The overall objective of the HumSat constellation is to provide messaging services through small user terminals on the basis of the store-and-forward concept. The project is open for international collaboration with organizations developing nanosatellites. The HumSat project is endorsed by the UN Program on Space Applications called BSTI (Basic Space Technology Initiative) which was started in 2009. 1)2)3)4)5)6)7)8)9)

The objective of this new educational project is to create a CubeSat constellation for the validation of GENSO (Global Educational Network for Satellite Operations). GENSO is a world-wide network of education and radio amateur ground stations linked together via the internet and using standard software developed under an ESA funded project. Through this network, GENSO provides operators of educational spacecrafts with extended satellite access capabilities.

GEOID (GENSO Experimental Orbital Initial Demonstration) is an ESA initiative and contribution to HumSat. The project is a first-class educational tool for engineering and science students providing additional benefits and opportunities for cooperation on a wider scale thanks to the worldwide dimension of HumSat. Twenty four (24) universities worldwide have already expressed their interest in contributing to the implementation of HumSat and out of these 24 universities, 19 have expressed their interest in developing the satellites of the global constellation. 10)11)12)13)

The GEOID initiative will be a GENSO (Global Educational Network for Satellite Operation) testbed for HumSat compatible nanosatellites. The GEOID project will act as a primary tool for the large-scale validation of the GENSO network to support future education satellite constellation operations.

The system is supported by a number of international organizations and countries:

• UNOOSA (United Nations Office for Outer Space Affairs). HumSat will be considered under the United Nations Basic Space Technology Initiative (BSTI).

• ESA (European Space Agency)

- Optional Educational Initiative for State and Cooperating Members

- GENSO development and operations node selected

- Leading the implementation of the test-bed of GENSO with HumSat compatible satellites via GEOID

• NASA (National Aeronautics and Space Administration), USA

- Educational program to launch US/CubeSats

- Use of GENSO for their CubeSat projects

- Selection of the US/American GENSO node.

• IAF (International Astronautical Federation)

- Promoting the HumSat idea with the Heads of Agencies in the world to allocate regular access to space on a rotating bases for humanitarian nanosats.

For retrieving data from the HumSat satellites, the GENSO network of ground stations will be one of the core components of the data distribution system. Several universities from different ESA member states, Japan and USA are cooperating in establishing the GENSO network, whose second release (R2) is expected to provide the functionalities that the HumSat system will require. Operations teams of the different satellites shall access their spacecraft using GENSO, through the SGI (Satellite-GENSO Interface). Teams that develop and construct their own ground station facilities shall also combine the operation of their satellites with the use of the GENSO network for improving the performance of the system, when it comes to downloading data from the satellites (Ref. 7).

Once data has been transported by the HumSat satellites, authorized users will be able to access it through an Internet connection by using the UHI (User-HumSat Interface). Several security restrictions shall be applied to guarantee private access to the gathered data. Figure 1 provides an overview of the principal elements HumSat system concept.

The general architecture of HumSat consists of the following segments: 14)

• Space segment, which is composed of several constellations and satellites which may act as a communications backbone for the system, offering coverage for the users/sensors to send data to the users/sensors.

The satellites of the constellation follow mainly the CubeSat standard with the sizes of 1U to 3U. The satellites will operate individually on the VHF, UHF or S-bands, according to each specific mission design.

• Ground segment, that will be used mainly for operating the satellites and for sending data to and from the user segment. This segment will be based in the extensive use of the services provided by the GENSO network (for communicating Emergency beacon localization, for providing support in humanitarian initiatives or in emergencies (natural disasters, accidents, etc.).with several satellites) and by the services of standalone ground stations developed, on their own, by additional users.

The ground segment will consist of monitoring stations that are able to work on the VHF, UHF and S-bands. These will be the control stations of the small satellite constellation and will be integrated into the GENSO network ito provide a worldwide coverage.

• User segment, formed by the sensors freely deployed and developed by users and by the facilities that users shall design and construct by their own in order to retrieve the data from the ground segment of HumSat through the Internet-based interface.

IGCE (Inexpensive Ground Communications Equipment): The IGCE or sensor segment is needed by the user community to be able to interconnect to the global system. The IGCE will provide a standardized interface for the transmission of humanitarian information. In this case between the sensors and the final transmission equipment. Different IGCE versions will be enabled depending on the mission type, the target area, the energy availability as well as the size and amount of data to be sent through the HumSat network.

HumSat service spectrum: The main goal of the HumSat system is to provide a generic communications service commonly known as “store-and-forward”, on which the different users of the system will be expected to build and define their own applications.

HumSat is a multi-purpose constellation. This is not only another attribute for the system, but a key concept that will enable the addition of other payloads to test on-orbit commercial systems developed by other companies. In this way, any other institution that wishes to perform on-orbit testing of spacecraft components can include them in these spacecraft.

Through an agreement with ESA, the IEC (Initial European Contribution) - GEOID to the HumSat system will be used as the initial testing tool of the GENSO network of ground stations. Several “key applications”' can be identified as the core set of applications for which the HumSat system is expected to provide its services:

• Communications support for short-messages exchanged in humanitarian aid missions, especially for regions where neither telecommunications nor electrical infrastructures have been constructed or where those have been damaged.

• Autonomous measurements of ECV (Essential Climate Variables), for instance, in situ measurements of atmospheric parameters, e.g. surface air temperature or wind speed and its direction, oceanic parameters (by fitting sensors to buoys or boats); sea-surface temperature or salinity; and terrestrial soil moisture. Since the network of sensors can be extended on demand, several new components can be added in different parts of the world for improving these measurements.

Table 1: The European universities participating in GEOID/HumSat with their developments and contributions to the system

HumSat operational scenario: GEOID and GENSO

Apart from being the initial European contribution to the HumSat system, the IEC contribution (GEOID) will become, through an agreement between ESA and the promoters, the test-bed constellation that will be used by ESA’s Education Office for testing the initial deployment of the GENSO network.

The HUMSAT community will, therefore, take advantage of the initial operative deployment of the GENSO network, and the GENSO network will be tested by commanding the spacecraft of the GEOID contribution. This win-win approach for the collaboration between the ESA Education Office and the HumSat Community is the so-called GEOID initiative.

To be able to perform the engineering and management tasks that the GEOID initiative will require, ESA has selected UVigo (University of Vigo) Spain as the system coordinator of the project. Since this is an educational project, and UVigo is an academic institution from an ESA-member state. ESA will provide engineering support for the tasks related to the HumSat system specification, and GEOID initiative.

Internal management and engineering approach: The design of the HumSat components that the GEOID initiative requires for GEOID will be based on the extensive use of the recommendations given by the ECSS (European Cooperation for Space Standards) space management and engineering standards.

The design of the space system will undergo a process that is based on the release, following the previously stated customer-supplier chain, of a set of specifications developed by customers. Afterwards, suppliers are expected to provide subsystems that meet the requirements of the previous given specifications. Customers shall finally verify the compliance of the delivered product with the specification previously released. These are the basis of the loop design recommended by the ECSS standards and represented in Figure 2, the so-called Systems Engineering V Model.

In the case of the GEOID contribution, the project definition is being carried out by UVigo (with engineering support from ESA), and will be based on the definition of the constellation parameters, assessment of its performance and further proposal of a baseline design of the CubeSat that will be developed by UVigo. This will form the UVigo baseline design that will be released along initial phases of the project.

The other universities that will join the GEOID contribution, shall use this design as the baseline for their own development; adapting it to its specific mission requirements, available funds, available human resources, and schedule constraints imposed by the given master schedule.

In summary, the GEOID contribution will be the Initial European contribution to the HumSat system and will develop the initial communications backbone for the communications network.

• A free non-guaranteed service of store-and-forward data for non-commercial uses

• Initial constellation set up mostly, but not exclusively, by satellites based on the CubeSat standard, mainly by universities and their students gaining hands-on space project experience

• Main applications will be non-commercial, mainly scientific and humanitarian support

• Ground segment mostly based on the use of the global GENSO network

• HumSat/GEOID and GENSO are open for participation to any interested non-commercial organization worldwide

Table 2: Overview of top-level HumSat characteristics

System-level interfaces:

The interconnection of all these components is done through the following interfaces:

• SSI (Space-to-Sensor Interface): SSI is the standard interface between sensors deployed by users and spacecraft. It provides communications capabilities in the UHF amateur satellite band at 437 MHz. The definition of this interface includes the definition of the parameters for the physical link and a custom MAC (Medium-Access) protocol for guaranteeing communications among spacecraft and sensors in a shared medium. This interface includes a new development of a protocol for the data uplink and downlink between satellites and sensors.

• SGI (Space-to-Ground Interface): SGI is the communications interface between spacecraft and the ground station that is defined by each spacecraft development team. In some cases, like the spacecraft developed by the University of Vigo, it will be based on ECSS (European Cooperation for Space Standards) tailored to the needs of the project. In case GENSO ground stations are used, this interface shall also meet all those requirements for the spacecraft to remain compatible with the ground stations of the GENSO network.

• UHI (User-to-HumSat Interface): UHI is an interface that will allow users to access to the services provided by the HumSat system through the internet. Including secure access and guaranteeing the data confidentiality for each user.

• Satellite Control Center-to-Ground Station Interfaces: These will allow spacecraft operators to connect to their own ground stations, as well as to GENSO remote stations. It will be based either on the use of the internet or a local area network for interconnecting these two facilities within the same university. For connections established with remote ground stations in the GENSO network, its specific interface requirements will have to be implemented in order to obtain the full-duplex real-time audio link requirements, as well as offline downlink capabilities.

• HumSat Payload Control Facility-to-Satellite Control Center Interfaces: These define the data exchange between each of the Satellite Control Centers for each of the spacecraft with the central HumSat Payload Control Facility (in Vigo). Data gathered by spacecraft from sensors is downloaded by each of the Satellite Control Centers and forwarded to the Payload Control Facility. Also, the data to be delivered to the sensor that needs to be uplinked using one of the spacecraft will be sent from the Payload Control Facility to the corresponding Satellite Control Center for the designated spacecraft. This interface is standardized and the same for all satellite developers.

Spacecraft development framework:

The availability of different resources and development times employed by each of the participating universities imposes the scalability of the system and an incremental service provision as a design driver for HumSat. This requires that the addition of new system components do not impact the functionality and performance of the existing ones. Based on this need, one of the objectives of HumSat is to establish a development framework for universities to independently design spacecraft, ground stations and/or sensors that could be added to the system, preserving its compatibility and increasing its overall performance. The spacecraft of ESA’s GEOID initiative will provide a demonstration of the system as well as being the European contribution.

Hence, the development framework to be established will consist of an initial top-level design into which different heterogeneous components with different schedules could be fitted:

1) Spacecraft are expected to be independently developed by different universities, and launched according to their own schedule and in the orbit they can afford. Minimum interface requirements will be imposed on them.

2) Each of the development teams is expected to develop its own ground station and they will be encouraged to add their ground station to the GENSO network. For this purpose, the specific interface requirements allowing for remote service provision will need to be implemented in the station control systems.

3) The sensors will be deployed independently by the users. The spacecraft interface will be imposed, but they are free to develop the applications, mange their use, select their location and define the amounts of data transmitted or received.

The constellation formed by spacecraft compatible with HumSat will not be a pre-designed one, but a set of spacecraft freely launched in different flight opportunities without pre-defined orbital positions or orbit control system. This is because CubeSats do not have propulsion and rely, for cost reasons, on piggyback launches whose orbits are defined by their primary passengers.

IGCE (Inexpensive Ground Communications Equipment)

The IGCE will provide a standardized interface for the transmission of humanitarian information. In this case between the sensors and the final transmission equipment. According with the mission, the target area, the energy availability as well as the size and the amount of data to be sent through the HumSat network, different IGCE versions will be enabled.

• A sensor segment or final data measuring equipments. These sensors or equipment will be connected through a standardized interface to the IGCE to allow immediate communication with any control and coordination center in the world (hospitals, supporting organizations, ...) once the small satellite has contacted the IGCE.

• UVigo developed a sensor prototype for IGCE. The cost of the final sensor is expected to be less than $15.

Figure 3: Illustration of a sensor prototype (image credit: UVigo)

• HumSat project will promote international cooperation and education.

• The small satellite constellation will be mainly based on the CubeSat standard but open to other standards as well.

• Each participating institution could participate in different ways:

- Developing a spacecraft and/or

- Sharing a ground segment and/or

- Providing sensors.

There are many possible applications of the HumSat constellation:

- Public health: Transmission of low data rate medical data.

- Monitoring and early warning for natural disasters through the use of ground sensor networks.

HumSat-D is a standard CubeSat of 10 cm side length and a mass of 1 kg designed and developed at the University of Vigo (UVigo), Spain. HumSat-D is the first (hence, a demonstrator) satellite of the future HumSat constellation. 15)16)

• The main mission is educational: to provide a hands-on experience to the students in the complete process of developing a space mission.

• The other goal is to demonstrate the validity of the concept of HumSat. A new subsystem to collect data from sensors located in the ground segment, store on-board and transmit it to ground stations will be developed and validated in orbit.

Spacecraft:

For the initial deployment phase of this system, the University of Vigo is developing a space system based on the CubeSat standard, for testing and validating in orbit the communications concept proposed for the HumSat system. This system is the so-called HumSat Demonstrator, and will be composed of a 1U CubeSat spacecraft, a typical Ground Station for CubeSats and a set of testing sensors. These testing sensors will be used for the proof-of-concept of the HumSat communications scheme (Ref. 15).

The spacecraft is manufactured by UVIGO and shall be capable of transporting data gathered from any sensor deployed by a customer either to UVIGO Ground Station or to a Ground Station developed by UVIGO but installed at customer’s facilities. This spacecraft is based on the design of the Xatcobeo CubeSat (launched on Feb. 13, 2012).

UVIGO shall also manufacture the Ground Station which will be placed at UVIGO’s facilities. The design of this Ground Station shall be based on the design of the Ground Station for the Xatcobeo project. This Ground Station shall be capable of commanding the spacecraft and of downloading of the data gathered from its sensors. The Ground Station shall also be capable of scheduling operations for the spacecraft to transmit the data gathered from a customer’s sensors while the spacecraft passes over the customer’s Ground Station.

UVIGO shall design, manufacture and install a Ground Station at customer’s facilities. This Ground Station shall only permit customer’s operators to receive the data gathered by the UVIGO spacecraft from this customer’s sensors. UVIGO shall manufacture a set of sensors which will be deployed by UVIGO for testing the proper functioning of the UVIGO spacecraft. The customers will also be allowed to develop and deploy their own sensors. Moreover, this mission shall be oriented only to “demonstrate” the capacity of building such a system like the one specified for the HumSat/GEOID mission. Therefore, the HumSat/GEOID mission and system requirements shall be tailored to these specific mission requirements.

The HumSat-D spacecraft will also carry a secondary mission in which it will measure the DDD (Displacement Damage Dose) along its orbit. This functionality will be implemented in a payload to be provided by INTA, with the same characteristics and configuration than the one provided for the Xatcobeo spacecraft. Data gathered from this experiment shall also be provided to INTA through the Internet.

In the fall of 2012, the HumSat-D spacecraft is in the Integration and Verification phase (Phase D, in accordance with ECSS definition standards) and is undergoing the vibration and thermo-vacuum tests at ESA facilities in Toulouse in the following month (October 2012).

Figure 5: Photo of the HumSat-D CubeSat during integration at the University of Vigo (image credit: UVigo)

The electronic boards (Figure 6) have been being integrated in the CubeSat. The one-way testing sensors are going to be deployed for testing the communications link with the CubeSat and, therefore, for validating the communications protocol proposed.

HumSat-D spacecraft:The CubeSat structure is based on Pumpkin's CubeSat kit with a side length of 10 cm and a mass of ≤ 1 kg.

ADCS (Attitude Determination and Control Subsystem): An ADCS is not needed due to the absence of any pointing requirements for the mission. The mass saving is used for the extra shielding needed in the higher elliptical orbit.

The OBDH (On-Board Data Handling) subsystem is a distributed system consisting of an OBC (On-Boar Computer) based on a Virtex-II FPGA, and OBPIC (On-Board Programmable Interface Controller) used to regulate the payload power and to condition the bus system. The onboard data are communicated via I2C bus.

Figure 8: Illustration of the OBDH (image credit: University of Vigo)

EPS (Electrical Power Subsystem) is provided by Clyde Space (UK) providing ~ 3 W of power from triple-junction solar cells. The solar cells are provided by Spectrolab and are referred to as UTJ (Ultra Triple Junction). The Li-ion battery has a capacity of 1250 mAh.

RF communications: The UHF (437 MHz) band is used in the uplink, and the VHF (145 MHz) band in the downlink. Use of a TNC (Terminal Node Controller) and 4 monopole (turnstile) antennas with omnidirectional radiation capability. The transponder works in half-duplex fashion using Manchester pulses (SP-L) for the downlink, and a phase with data subcarrier (PM/PBSK) for the uplink. Use of the CCSDS protocols for frame and channel coding. A data rate of 1.2 kbit/s is used in the uplink. The downlink has a data rate of 9.6 kbit/s.

Launch: The launch of HumSat-1 as a secondary payload is scheduled for the first half of 2013 on a Dnepr-1 launch vehicle from the Dombarovsky launch site, Russia. The launch provider is ISC Kosmotras.

HumSat-D will be integrated into the UniSat-5, a microsatellite of the University of Rome (La Sapienza), and deployed by PEPPOD after orbit injection.

The objective of the DCS is to receive, store and retransmit information collected from globally located in-situ measurement sensors in the ground segment. The DCS transmits the collected information to a GENSO ground station, which in turn transmits the information via Internet to the UVigo server.

The DCS permits redundant and parallel data reception links from up to 4 PTTs (Platform Transmitter Terminals) of the various clients in the ground segment, equipped with sensors to measure environmental parameters.

Note: The approach of the HumSat DCS system is similar to the Argos system flown on the NOAA POES (Polar Orbiting Environmental Satellite) series and on the Argos system flown on the MetOp series spacecraft of EUMETSAT.

The RDDDS is an INTA provided device developed in the INTA Electronics Design Laboratory. RDDDS is an upgrade version of the ODM (OPTOS Dose Measurement) payload developed for the OPTOS nanosatellite mission of INTA (launch planned for spring 2010). The sensors are being provided by CNRS/LAAS (Laboratory for Analysis and Architecture of Systems) of Toulouse, France. The objectives of RDDDS are to measure TID (Total Ionizing Dose) of the incoming radiation to improve the current space environment models.

16) Information provided by Fernando Aguado of the University of Vigo, Vigo, Spain

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.